There has been known a piezoelectric oscillator which includes an auxiliary oscillator having an auxiliary electrode provided separately from a main electrode on a piezoelectric substrate, detects a frequency of an oscillation output signal of the auxiliary oscillator, and estimates a temperature of a crystal unit based on the frequency and the temperature characteristics of the frequency of the auxiliary oscillator.
In recent years, in an effort to meet the requirements of reduction in device size, compactness and high-density packaging of parts and modules are in progress. With no exception, the downsizing of the crystal units serving as clock sources have also been in progress. Under such circumstances, when it is considered that a function failure has occurred in a device due to an abnormality of a crystal unit, it is useful to provide the ability to inspect the electrical characteristics of the crystal unit in a state where it is packaged into the device. This is because it is difficult to take out and inspect only the crystal unit for the reason that peripheral parts may be damaged when the crystal unit is removed from the device in a high-density packaging state.
The following is a reference document.    [Document 1] Japanese Laid-Open Patent Publication No. 2011-135342.